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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

LUCIA AND ATBJEVIDA GLACIERS                              61
For two miles or more the outer margin of the Lucia lobe was clothed with vegetation. On the inner side of this forest-covered part of the ablation moraine scattered individuals and clusters of alder were growing in the more stable portions of the moraine, but the amount of vegetation rapidly increased toward the outer margin of the glacier, and in a short distance the entire surface was covered and hidden by alder thickets. Farther out individual cottonwoods rose above the alder, and at the extreme margin a forest of cottonwood and spruce occupied the ablation moraine. The forest covered more than a mile of the outer glacier margin and in it no ice was visible from a distance. The extreme outer margin consisted of a low, wooded, gently-sloping embankment which rose above the gravels of the Kwik valley, and it was exceedingly difficult in places to tell where the glacier ended. Without examining it carefully, one unfamiliar with the phenomenon of forest-covered glaciers would not suspect that this wooded slope was a glacier end. One noticeable feature, however, was the large number of small, trickling streams of clear, cold water that emerged from the embankment, and at one or two points larger streams, bearing sediment.
In the ablation moraine were many interesting details, most of which have no direct bearing on the glacier as a whole but are phenomena of melting. There were, however, several noteworthy features. The most prominent of these was the extreme irregularity of the surface due to differential melting. In crossing the ice surface one was constantly climbing over knolls and ridges and descending into valleys, some roughly circular, others linear or crescentic, and the difference in elevation from valley bottom to ridge crest was often fully 100 feet. The elevations occurred where the drift was thick, the depressions where it was thin, and it was evident that the relative positions of these higher and lower portions of the moraine surface were constantly changing as the thickness of the drift cover changed through downsliding. During the ablation of the under ice, steep slopes are developed down which the debris slides to the depressions, leaving nearly or quite bare ice slopes, while the de'bris accumulates in the depressions and in time doubtless accumulates to sufficient depth to retard their further reduction. Then the depressions are gradually transformed to elevations, and the former elevations, denuded of their drift cover, are lowered to valleys. Where the drainage escapes into moulins there are conditions favoring more permanent valleys. It is this constant shifting of de'bris that prevents the growth of vegetation on these thinner portions of the ablation moraine, and the struggle that plants are making to find a foothold here is often illustrated near the margin of the plant-covered portion of the ablation moraine, where annual plants and one or two year old alder and willow bushes are found overturned, or dying where they stand by the removal of the soil from around their roots.
A second noteworthy feature of the ablation moraine was the fact that the rock fragments were all angular, so far as we were able to observe. They were frost-riven fragments and avalanche falls derived from the steepened upper valley walls and in part, doubtless, supplied with the snow itself as it slid down into the reservoirs. In the outer part of the piedmont bulb the ablation moraine consisted of a confused maze of hillocks and depressions, with no definite system, and with a topography reminding one of some areas of sand dunes where ridges are absent; but farther up the glacier, where the moraine was thinner, the ablation moraine presented a series of crescentic ridges, where longitudinal ridges, perhaps medial moraines, swung across the glacier instead of down it. The cause for this peculiar arrangement of the morainic ridges is not clear, but is evidently